[PATCH] slim: fix bug with mm_users usage

[PATCH] slim: fix bug with mm_users usage

[Kylene Jo Hall]

There is a NULL pointer dereference possible that was introduced in the
last round of modifications to the demotion code before merging.
current->mm should be checked for existence before it is dereferenced to
check the value of the mm_users field.  This patch fixes all instances
of this bug.

Signed-off-by: Kylene Hall <kjhall@us.ibm.com>
---
 security/slim/slm_main.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

--- linux-2.6.18-rc6-orig/security/slim/slm_main.c	2006-09-18 16:41:51.000000000 -0500
+++ linux-2.6.18-rc6/security/slim/slm_main.c	2006-09-22 13:58:35.000000000 -0500
@@ -529,7 +519,7 @@ static int enforce_integrity_read(struct
 	spin_lock(&cur_tsec->lock);
 	if (!is_iac_less_than_or_exempt(level, cur_tsec->iac_r)) {
 		rc = has_file_wperm(level);
-		if (atomic_read(&current->mm->mm_users) != 1)
+		if (current->mm && atomic_read(&current->mm->mm_users) != 1)
 			rc = 1;
 		if (rc) {
 			dprintk(SLM_BASE, "ppid %d(%s p=%d-%s) "
@@ -1100,7 +1092,7 @@ int slm_socket_create(int family, int ty
 			memset(&level, 0, sizeof(struct slm_file_xattr));
 			level.iac_level = SLM_IAC_UNTRUSTED;
 			rc = has_file_wperm(&level);
-			if (atomic_read(&current->mm->mm_users) != 1)
+			if (current->mm && atomic_read(&current->mm->mm_users) != 1)
 				rc = 1;
 			if (rc) {
 				dprintk(SLM_BASE,
@@ -1306,7 +1298,7 @@ static int enforce_integrity_execute(str
 		cur_tsec->iac_r = cur_tsec->iac_wx;
 	} else {
 		rc = has_file_wperm(level);
-		if (atomic_read(&current->mm->mm_users) != 1)
+		if (current->mm && atomic_read(&current->mm->mm_users) != 1)
 			rc = 1;
 		if (rc) {
 			dprintk(SLM_BASE,

Re: [PATCH] slim: fix bug with mm_users usage

[Hugh Dickins]

On Fri, 22 Sep 2006, Kylene Jo Hall wrote:

Nothing to do with this patch as such, but as it went past I noticed

> --- linux-2.6.18-rc6-orig/security/slim/slm_main.c	2006-09-18 16:41:51.000000000 -0500
> +++ linux-2.6.18-rc6/security/slim/slm_main.c	2006-09-22 13:58:35.000000000 -0500
> @@ -529,7 +519,7 @@ static int enforce_integrity_read(struct
>  	spin_lock(&cur_tsec->lock);
>  	if (!is_iac_less_than_or_exempt(level, cur_tsec->iac_r)) {
>  		rc = has_file_wperm(level);
> -		if (atomic_read(&current->mm->mm_users) != 1)
> +		if (current->mm && atomic_read(&current->mm->mm_users) != 1)
>  			rc = 1;

I've not studied your patches, and I don't know what that line's about,
but you appear to be attaching considerable significance to the value of
mm->mm_users.  Yet swapoff (try_to_unuse) has to bump it up to hold the
mm temporarily, as does get_task_mm() used in various places (mainly /proc).

Hugh